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In normal subjects and patients with airway obstruction, flows during a forced vital capacity (FVC) manoeuvre are higher after a fast inspiration without an end-inspiratory pause (manoeuvre 1) as compared to a slow inspiration with an end-expiratory pause of ~5 s (manoeuvre 2). In this study, we investigated the influence of these manoeuvres on maximal expiratory volume-time and flow-volume curves in patients with restrictive lung disease.Eleven patients with restrictive lung disease were studied. Their average (±SD) lung function test results were: FVC=55±12% predicted value, forced expiratory volume in one second (FEV1) 52±20% pred, FEV1/FVC 85±6%, total lung capacity 55±8% pred, and carbon monoxide transfer factor 47±18% pred. The patients performed the two FVC manoeuvres in random order. We compared the ensuing spirograms and maximal expiratory flow-volume curves from which peak expiratory flow, FEV1, FEV1/FVC, maximal mid-expiratory flow, and maximal flows were computed.All spirometric indices were significantly higher with manoeuvre 1 than 2. Maximal expiratory flows at the same lung volume were also significantly higher with manoeuvre 1 than 2, in all patients.Routine spirometric indices, obtained during a forced vital capacity manoeuvre depend on the time course of the preceding inspiration in patients with restrictive lung disease. Therefore, the forced vital capacity manoeuvre should be standardized if used in clinical, epidemiological and research studies.

Background-The measurement of effective alveolar carbon dioxide tension (PACo2eff) is still a matter of debate. It has, however, become common practice to use arterial instead ofalveolar CO2 tension for computing alveolar oxygen tension (PAo2) and physiological dead space, not only in normal subjects but also in patients. The purpose of this study was to estimate alveolar CO2 tension during spontaneous breathing with a new bedside technique which is simple and non-invasive, and to compare these values with arterial CO2 tension measured in normal subjects and patients with chronic airways obstruction. Methods -The subjects breathed quietly through the equipment assembly (mouthpiece, monitoring ring, Fleisch transducer head) connected to a pneumotachograph and a fast response infrared CO2 analyser. The method is a computerised calculation of the volume weighted effective alveolar CO2 tension obtained from the simultaneously recorded expiratory flow and CO2 concentration versus time curves. An arterial blood sample was taken to measure PaCo2 for comparison during the study. Results -The results showed a mean difference (PAco2eff-Paco2) of -0 205 kPa in 20 normal subjects and -0'460 kPa in 46 patients. The 95% confidence interval of the bias was -0-029 to -0*379kPa in normal subjects and -0*213 to -0 707 kPa in patients. The limits of agreement between PAco2eff and PaCo2 were 0*526 to -0 935 in normal subjects and 1170 to -2-088 in patients. Conclusions -The volume weighted effective alveolar Pco2 in normal subjects and patients with chronic airways obstruction is lower than the arterial Pco2 and is recommended as a better estimate in the classical equations for estimating dead space and intrapulmonary shunt. (Thorax 1995;50:240-244) Keywords: alveolar CO2 tension, alveolar-arterial Pco2 difference, chronic airways obstruction, spontaneous breathing.The estimation of effective alveolar carbon dioxide tension is still a matter of debate because the available methodology is not widely accepted. In most classical calculations alveolar carbon dioxide tension (PACo2) has been assumed to be equivalent to arterial blood CO2 tension (Paco2) in normal subjects.' '°It has also become common in clinical practice and in research work to use arterial CO2 tension as a substitute for alveolar CO2 tension for the computation of alveolar oxygen tension (PAo2) and of the physiological dead space/tidal volume ratio (VDphysNVT) .l2 56 2 11 The aim of this study was to estimate effective alveolar CO2 tension during tidal breathing with a new and simple technique and to compare these values with arterial CO2 tension in both normal subjects and patients with chronic airways obstruction. Methods THEORETICAL CONSIDERATIONSThe method is based on the computation of the effective CO2 concentration in the expired air at the mouth (FECO2efI) and the dead space/ tidal volume ratio (VD/VT) on the VE/time

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